Resizing + tweening can make the background clear color visible

[akien-mga]

The resizing code has been improved a lot to fix #79, but there are still some smaller issues when resizing the window in all directions like you're trying to find a bug :D

See e.g., after resizing several times to a very small size and back to a big size:

The background clear color becomes visible, i.e. the ParallaxBackground is not properly scaled.

[bojidar-bg]

@akien-mga Does it happen when you resize it slowly?

[ObaniGemini]

This doesn't seem to happen anymore